Mixed flock between 19 and 20 weeks of age, no eggs yet, but they look very close. They have a clean, enclosed coop with pine bedding (just cleaned and put in fresh a few days ago), great ventilation, enclosed run and get out to free range a couple hours a day. No symptoms of any kind (that I can see) in the flock. No runny eyes or nose, no cough or sneeze, no funny poop, no mites, ticks, fleas, or other cootie, great appetities, very active. A couple of birds have some swelling around the eyes, but it looks to be pecking trauma. Info on hen in question: barred rock, just short of 20 weeks, just changed over to layer feed 2 days ago, no trauma or other obvious injury, no runny eyes or nose, no off coloration, only odd behavior was that I found her panting under the coop (weather was cool), and her crop was empty at bedtime. She was hesitant to go up on the roosts last night. She didn't have any swelling anywhere, no mites, no other parasites, no cough or sneeze. The only thing I did notice was that she had poop on her back fluff and it was light in color and looked a bit chalky. I noticed the poop there two days ago, but she seemed fine otherwise so I cleaned it off a bit and let her back with the rest.could this be an eggbound hen? Could it be some poison? There doesn't seem to be anything around she could have gotten into (except for some of the old barn foam insulation, but I covered that up weeks ago.) I have no dea what killed this sweet little pullet and I don't want to lose any more. This is my first time with chickens.please help!!!